Hardcoded namespace name in deployment manifest

The [install CRD](https://github.com/argoproj-labs/appsource/blob/master/docs/GETTING_STARTED.md#install-appsource-crd-and-controller) installation step as per "Getting started" doc fails due to the deployment namespace being set to "system" in the [install.yaml](https://github.com/argoproj-labs/appsource/blob/afe743a870bf836b9e2bc8f539b9c74e0714f9ed/manifests/install.yaml#L411)

Should the line specifying namespace be removed from [install.yaml](https://github.com/argoproj-labs/appsource/blob/master/manifests/install.yaml#L411) and [deployment.yaml](https://github.com/argoproj-labs/appsource/blob/master/manifests/deployment/deployment.yaml#L5)?

Start with the installation step in docs/GETTING_STARTED.md and inspect the namespace entries at the cited lines in manifests/install.yaml and manifests/deployment/deployment.yaml. Reproduce the CRD and controller installation to confirm the failure. Done means the deployment no longer depends on the hardcoded "system" namespace and both manifests install consistently.

Written by the indexing model from the issue text.